The present invention generally relates to polymer compositions having high thermal conductivity and dielectric strength. Particularly, the polymer composition comprises a base polymer matrix and a thermally-conductive, electrically-insulating material. The invention also encompasses molded packaging assemblies for electronic devices such as capacitors, transistors, and resistors. The invention further includes methods for making such packaging assemblies.
Electronic devices such as semiconductors, microprocessors, circuit boards, capacitors, transistors, and resistors can generate a substantial amount of heat that must be removed in order for the device to function properly. For example, a 1000-Ohm resistor in a 120 Volt circuit must dissipate 14 Watts of energy during operation. If the resistor is installed in a densely packaged circuit board, it is highly susceptible to overheating that can destroy the resistor and other components in the board. The industry has attempted to address this problem in a variety of ways.
For example, the electronic device may have a metallic outer cover. The cover can be made from aluminum or copper and installed onto the device. Although a metallic cover can dissipate a substantial amount of heat, it is electrically-conductive. Thus, an electrically-insulating layer must be placed between the device and cover. Although the electrically-insulating layer provides good electrical isolation, it is generally a poor thermal conductor. Thus, the electrically-insulating layer can prevent effective heat conduction between the device and device cover.
It is also common to employ heat pipes having a metal casing to help remove heat from a heat-generating object. In addition, metal heat spreaders in a laptop computer may be employed for transferring and dissipating heat. While these metal components can effectively dissipate heat and have good mechanical strength, they are typically electrically-conductive. Therefore, these components must be electrically insulated from the heat-generating components so as not to interfere with their electrical operation.
The industry uses thermally-conductive polymeric compositions in an attempt to overcome some of the negative aspects found with metallic heat transfer components. These thermally-conductive polymeric compositions have some advantages over metallic components. For example, the polymeric compositions can be injection-molded into parts having complex geometries such as interface pads which are placed between the heat-generating device and heat sink. The injection-molding process is effective, because it can produce a “net-shape” part. The final shape of the part is determined by the shape of the mold cavity. No further machine tooling is required to produce the final shape of the part. In contrast, such additional machine processing is often needed for shaping metallic parts, and this processing can be costly and time-consuming. In addition, polymer materials are often lighter and less costly than metallic parts.
McCullough, U.S. Pat. No. 6,251,978 (the '978 patent) discloses a thermally and electrically-conductive composite material that is net-shape moldable. The '978 patent discloses a composition containing: a) between 30 to 60% by volume of a polymer base matrix, b) between 25 to 60% by volume of an electrically-conductive filler having a relatively high aspect ratio of at least 10:1, and c) between 10 to 25% by volume of an electrically-conductive filler having a relatively low aspect ratio of 5:1 or less. The '978 patent discloses that the materials employed for the high aspect and low aspect ratio fillers may be selected from the group consisting of aluminum, alumina, copper, magnesium, brass, and carbon.
As described in the '978 patent, fillers used in conventional polymer compositions are typically thermally and electrically-conductive. For example, when a polymer base matrix is loaded with carbon fibers and metallic flakes to enhance the thermal conductivity of the composition, the composition is also quite electrically-conductive. As a result, parts made from thermally-conductive polymer compositions also generally require an additional electrically-insulating layer when placed in contact with electrically-conductive, heat-generating devices.
In view of the foregoing problems, it would be desirable to have a polymer composition with high thermal conductivity as well as high dielectric and mechanical strength. Such a composition should be capable of being molded into a packaging assembly for electronic devices. The present invention provides such a polymer composition. The invention also encompasses molded packaging assemblies for electronic devices. The invention further includes methods for making such packaging assemblies.